<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>边框</title>
    <style>
        body{
            background-color: #ccc;
        }
        .trigle {
            width: 0;
            height: 0;

            border-top: 0px solid transparent;
            border-right: 50px solid transparent;
            border-bottom: 100px solid red;
            border-left: 50px solid transparent
        }

        .trigle2 {
            width: 0;
            height: 0;

            border-top: 0px solid transparent;
            border-right: 100px solid transparent;
            border-bottom: 100px solid red;
            border-left: 100px solid transparent
        }

    </style>
</head>
<body>

    <div class="trigle"></div>
    

    <div class="trigle2"></div>


</body>
</html>